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SubjectRe: [PATCH v29 00/20] Intel SGX foundations
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On Wed, 06 May 2020 17:14:22 -0500, Sean Christopherson  
<sean.j.christopherson@intel.com> wrote:

> On Wed, May 06, 2020 at 05:42:42PM -0400, Nathaniel McCallum wrote:
>> Tested on Enarx. This requires a patch[0] for v29 support.
>>
>> Tested-by: Nathaniel McCallum <npmccallum@redhat.com>
>>
>> However, we did uncover a small usability issue. See below.
>>
>> [0]:
>> https://github.com/enarx/enarx/pull/507/commits/80da2352aba46aa7bc6b4d1fccf20fe1bda58662
>
> ...
>
>> > * Disallow mmap(PROT_NONE) from /dev/sgx. Any mapping (e.g.
>> anonymous) can
>> > be used to reserve the address range. Now /dev/sgx supports only
>> opaque
>> > mappings to the (initialized) enclave data.
>>
>> The statement "Any mapping..." isn't actually true.
>>
>> Enarx creates a large enclave (currently 64GiB). This worked when we
>> created a file-backed mapping on /dev/sgx/enclave. However, switching
>> to an anonymous mapping fails with ENOMEM. We suspect this is because
>> the kernel attempts to allocate all the pages and zero them but there
>> is insufficient RAM available. We currently work around this by
>> creating a shared mapping on /dev/zero.
>
> Hmm, the kernel shouldn't actually allocate physical pages unless they're
> written. I'll see if I can reproduce.
>

For larger size mmap, I think it requires enabling vm overcommit mode 1:
echo 1 | sudo tee /proc/sys/vm/overcommit_memory


>> If we want to keep this mmap() strategy, we probably don't want to
>> advise mmap(ANON) if it allocates all the memory for the enclave ahead
>> of time, even if it won't be used. This would be wasteful.
>>
>> OTOH, having to mmap("/dev/zero") seems a bit awkward.


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